For approximately ten years, a large number of applications have a sample analysis requirement, whether in solid, liquid or gaseous form. Laboratories have developed a number of diagnostics tools allowing the composition of a sample to be determined to meet these requirements. The tools may use various chemical, physical, or even mechanical principles. They include, for example, the methods of plasma emission spectroscopy (ICP), spectrometry, electrochemistry, calorimetry, etc.
A large number of analyses are based on gas chromatography combined with known mass spectroscopy or plasma emission spectroscopy techniques. In spite of the efficiency of analysis tools in terms of detection threshold, the latter are, on one hand, very expensive and, on the other hand, not portable. They are installed in analysis laboratories, require very careful preparation of the sample, and need highly qualified staff to carry out the measurements and interpret the spectra. One analysis thus requires, on average, a period of three days between collection of samples and determining the result of its composition.
While using those diagnostics tools, the LIBS (Laser Induced Breakdown Spectroscopy) technology, invented in laboratories in 1989, has become over the last few years a means of analysis of the atomic composition of materials competing with the ICP. The LIBS may have the advantage of portability and lesser preparation of the sample. This allows the analyses to be carried out on site. The well known general principle of the LIBS technology is to analyze fluorescence emitted by the previously atomized sample. The analysis of ratios of emission lines allows a quantitative measurement of the concentration of the species in the material.
More specifically, a material, whether it is in solid, liquid or gaseous form may, after excitation by a laser, be transformed into plasma (mixture of free electrons, ions, atoms and molecules) resulting from the ionization, for example, by multi-photonic absorptions or by the tunnel effect. If excitation of the material is significant enough, other well known physical phenomena come into play such as cascade ionizations and collisions between free electrons. The effects increase the temperature of the plasma produced. The Bremsstrahlung of the moving electrons (inverse Bremsstrahlung effect) therefore gives a white light emitted by the plasma. Analysis of the radiative deexcitation of the atoms and ions therefore allows the latter to be traced back to the composition via a spectral analysis of the white light emitted by the plasma. The atomic lines having a much longer lifetime than the continuum of white light, a delayed detection of the spectrum allows the atomic lines of the spectrum to be isolated for tracing back to the composition.
Conventional laser sources used in that type of application are nanosecond YAG type laser sources with a 1,064 nm wavelength generating energy pulses on the order of a few tens of millijoules. Focusing the laser beam is carried out with the aid of a lens generally protected by an interchangeable protection window.
The detection and collection of the fluorescence are carried out according to known techniques with an optical fiber placed at the level of the plume of the plasma. The light transmitted by the fiber is sent into a spectrometer for detection by a CCD or ICCD camera accompanied by a full-size grating or more generally a monochromator. Recognition of the LIBS spectra requiring a good optical resolution (typically between 1,000 and 3,000), to be able to differentiate samples of similar composition on a broad spectrum, the existing systems use the following detection methods: a spectrometer equipped with a full-size grating with variable blaze, or a set of spectrometers in parallel, or a spectrometer equipped with a full-size grating and a prism.
The disadvantages of such detection systems are their cost, and low luminosity linked to the entrance slit making exploitation of the results difficult.
Known waveguide integrated spectrometers include U.S. Pat. No. 5,615,008. Such a system includes a waveguide inside of which Bragg gratings are placed to redirect the light from the waveguide outwardly of the guide. Such a system can operate as a spectrophotometer, spectrofluorimeter, or other means for analyzing the components of lights after the passage of a sample.
Once again, the luminosity and the resolution obtained by the type of spectrometer are not sufficient for certain element detection applications. The Bragg gratings inscribed to reflect the light outside of the fiber are not very effective due to the low thickness of the grating inscribed limited by the diameter of the optical fiber.
Moreover, the fact that the Bragg gratings are directly integrated into the optical fiber prevents the detection wavelengths of the gratings from being tuned because the angle of incidence on the Bragg grating of the guided light is fixed. The gratings inscribed in the fibers therefore have a number of practical limitations.
It could therefore be advantageous to provide a detection system which is wavelength tunable. It could also be advantageous to provide an element detection system allowing a compact system to be obtained, while maintaining good resolution and a large degree of luminosity.